What's That Noise? Furnace, AC & Water Heater Sound Decoder
Pick the system, pick the sound, and get a plain-language read on what it usually means and what to do right now. No contact details required — the answer shows right here.
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Furnace, AC & water heater sounds — what they usually mean
The full list, grouped by system, so you can read it even if you don't use the tool above.This is general information, not a diagnosis. Never remove panels on a gas appliance or open an electrical panel. If you're not sure, turn it off and call.
Furnace
- Loud bang or boom when it startsTurn it off and call
What it usually means: Can be delayed ignition, where gas builds up before it lights.
What to do: Turn it off at the thermostat and call. Don't keep restarting it.
- Squealing or screechingGet it looked at soon
What it usually means: Often the blower motor or its bearings.
What to do: Get it looked at soon.
- RattlingGet it looked at soon
What it usually means: A loose access panel, loose ductwork, or a loose part in the blower.
What to do: Make sure the furnace door is fully closed; if it continues, call.
- Clicking over and over, but no heatGet it looked at soon
What it usually means: The ignition is trying and failing to light.
What to do: Get it looked at soon.
- Humming or buzzingGet it looked at soon
What it usually means: Often a motor or electrical part.
What to do: Get it looked at soon.
- WhistlingGet it looked at soon
What it usually means: Usually restricted airflow.
What to do: Check the filter and make sure vents are open.
- Hissing near the gas line, or a rotten-egg smellTurn it off and call
What it usually means: A possible gas leak. Stop and follow the gas-safety steps below.
What to do: Smell gas? Leave the house now. Don't flip switches or use your phone inside. Once you're outside, call 911 or your gas utility's emergency line. After the utility makes it safe, call us to fix the problem.
Outdoor AC Unit
- Loud buzzing and the fan isn't spinningTurn it off and call
What it usually means: Often an electrical part.
What to do: Turn it off at the thermostat; running it this way can damage the compressor.
- Hissing or bubblingTurn it off and call
What it usually means: Can mean a refrigerant leak.
What to do: Turn it off and call.
- RattlingGet it looked at soon
What it usually means: Debris or a loose panel.
What to do: Turn it off before looking; call if it continues.
- ScreechingTurn it off and call
What it usually means: Fan motor or compressor trouble.
What to do: Turn it off and call.
- Clicking at startup, then nothingGet it looked at soon
What it usually means: Usually electrical.
What to do: Get it looked at soon.
Water Heater
- Rumbling or poppingGet it looked at soon
What it usually means: Usually sediment built up in the bottom of the tank, common in older tanks.
What to do: Get it looked at soon.
- Hissing or sizzlingTurn it off and call
What it usually means: Water may be dripping onto the burner or the tank may be leaking.
What to do: Check for water around the base and call.
- Water or steam coming from the relief valveTurn it off and call
What it usually means: A relief valve discharging water or steam needs attention right away.
What to do: Call right away.
- Light tickingOften normal
What it usually means: Pipes expanding and contracting as they heat and cool.
What to do: Often normal — no action needed unless it changes.
This is general information, not a diagnosis. Never remove panels on a gas appliance or open an electrical panel. If you're not sure, turn it off and call.
Related help
Once you know which system is acting up, here's where to go next.
Sound questions, answered
Plain answers about furnace, AC and water heater noises, and when a sound is a reason to stop and call.